FRP, in particular, CFRP (carbon fiber reinforced plastic), is utilized in various fields as a composite material having a property light in weight and high mechanical properties. As one of FRP molding methods, an RTM molding method is known wherein a reinforcing fiber substrate such as a laminated substrate of reinforcing fiber woven fabrics is placed in a mold, and after the mold is clamped, a liquid resin is injected into the mold reduced in pressure, and the resin is heated and cured. Further, in such a conventional molding, it is proposed to give a certain shape to a reinforcing fiber substrate before disposing it in a mold by clamping it with upper and lower preforming dies (for example, JP-A-2003-305719).
In a conventional RTM molding method, generally resin is injected at a pressurized condition from a single injection port. Then, as the case may be, a plurality of resin discharge ports are provided. In such a conventional method, however, there is a problem that RTM molding of a large product is difficult, because it is difficult to set a large amount of flowing resin and there is only one resin injection port. Namely, gelation of resin progresses (resin viscosity increases) during resin flowing, and there occurs a case where the resin does not flow over the entire area of a product to be molded. Further, if the time of gelation is extended by adding a delay agent to the resin, it is possible to flow the resin over the entire area although a long time is required, but too much time is required for achieving a predetermined resin flow, thereby decreasing the production speed and the production amount. Futhermore, when a large product, in particular, a relatively large three-dimensional plane-like product is molded, if resin is flown from a single injection port, in accordance with the shape, there may occur an area where the resin does not flow. Even if the resin flow is controlled by providing a plurality of resin discharge ports, there is a limit for molding a complicated structure properly.
On the other hand, as a method for carrying out resin injection at a time from the entire surface of a product to be molded, there is an RFI (Resin Film Infusion) method. In this method, non-impregnated reinforcing fiber substrate applied with a semi-cured resin film is heated, and molten resin is impregnated by pressing it by hot press and the like, but a complicated-shape molding is difficult, and there is a problem that a non-impregnated portion is liable to occur in a part of the reinforcing fiber substrate.
As a method of impregnation even for a product to be molded which is complicated to some extent and large, there is a method described in JP-A-2002-234078. In this method, a carrier of a matrix resin, for example, prepared by impregnating a molten resin into a sponge material, is used instead of the resin film in the aforementioned RFI method, and although it is an improved method, because a method for covering the entire product to be molded with a bagging film and reducing in pressure the inside thereof is employed as a method for achieving pressure impregnation for a large product in an inexpensive and easy manner, a pressurizing force of only 0.1 MPa can be generated at maximum, and therefore, there are problems that impregnation for a thick product is difficult and that impregnation up to detailed portions is difficult.
Since any of these methods is not a method for impregnating a matrix resin into a reinforcing fiber substrate while flowing the molten resin from initial time, there left a cause for generating non-impregnated portions.
Further, as a conventional RTM molding method, a method is also known wherein resin is injected at a pressurized condition from a single injection line. For example, when a product to be molded has a shape of a polygon (a shape with a plurality of sides), the resin is injected from a certain one side toward another side opposite to the certain one side (for example, JP-A-8-58008 and JP-A-2003-11136). In such a method, however, although the resin surely flows from one side toward the opposite side while the resin is impregnated into a reinforcing fiber substrate in order, if the product to be molded is relatively large, it takes much time to flow the resin, and as the case may be, the resin may reach a time of its gelation during its flow, in such a condition there is a problem that the resin flow stops before complete impregnation. Accordingly, as described in the aforementioned JP-A-8-58008, a method for providing resin injection lines at a plurality of positions of a product to be molded and injecting the resin in order is proposed. In this method, however, since the resin is injected from portions within a molding area of the product to be molded, it cannot be applied to a sandwich molded product using a core material and disposing reinforcing fiber substrates on both surfaces of the core material, because the resin cannot be injected from a mold surface side. Further, even in a case of a non-sandwich molded product, the method cannot be applied to the molding of a product which is double-sided and which requires a high design quality for its surface. Thus, in the above-described conventional RTM molding methods, it is difficult to efficiently mold a relatively large product.
Usually, in an RTM mold consisting of relatively many dies, there is a big problem that the productivity is low, because the molding takes much time. On the other hand, in a structure of a mold consisting of an upper die and a lower die, although it has an advantage that the aforementioned setting of a reinforcing fiber substrate onto the surface of the mold is relatively easy and the setting time is short, in a case of a general resin injection method, that is, in a case where resin is pressurized at a pressure of 0.2 to 1.0 MPa and the resin is injected without a particular control of flow rate, the resin flows into the mold at a flow rate corresponding to the pressure and the resin is charged in the mold in a relatively short period of time, but there may occur a case where the reinforcing fiber substrate is disturbed by the resin flow, or where a non-uniform flow occurs by a high flow rate and many voids and pinholes are generated on the surface of a molded product.
In particular, in a case where resin injection is carried out at a high resin discharge pressure of 0.5 MPa or more (therefore, at a high flow rate) to shorten a molding time or to mold a product having a large area in a short period of time, disturbance of the weave structure of a reinforcing fiber substrate (particularly, a plain weave fabric) is liable to occur, and further, because the resin flows in the mold at a high speed, the resistance against the flow disperses within the flow area depending upon a dimensional unevenness (particularly, an unevenness in thickness) of a cavity in the mold, a fine unevenness in thickness of the substrate, or a difference between partial structures of the substrate due to overlapping of substrate layers and the like, and because a uniform flow cannot be maintained, there is a case where a large void is generated by occurrence of a local forestalling of the resin flow and the like. Furthermore, there is a case where the resin actually flows up to the substrate portion, but, because the flow rate is high, for example, there is no time for release of gas present in the texture of the woven fabric and the gas stays there, and the gas generates a surface defect such as a pinhole. In such conventional molding conditions and molding process causing reduction of quality in appearance concerning the design quality such as substrate disturbance, voids and pinholes, it is difficult to ensure a high surface quality while carrying out a high-speed injection for shortening the molding time. The larger the size of a product to be molded becomes, the more frequently such defects on quality in appearance are liable to occur, because a high-speed resin injection is to be inevitably employed.
Because the flow state of resin greatly influences generation of such voids and pinholes concerning design quality, the density of the reinforcing fiber substrate, that is, the weight thereof, also becomes an important factor. Namely, because a weight of reinforcing fibers per one layer influences a flow resistance of resin and easiness of gas release, it is necessary to set a proper weight in accordance with the resin flow condition. This proper weight has to be set from the viewpoints of not only the surface quality but also the workability and utilization factor in strength of a preform. Namely, if the weight is too great and the rigidity of the substrate becomes high, the reinforcing fiber substrate becomes hard to be situated along the mold surface and hard to be formed in a three-dimensional shape, and there is a case where it takes much working time to make a preform, or that at that time disturbance of the substrate occurs and the mechanical properties of the FRP molded product decrease. Namely, to carry out an efficient production, there is a proper weight corresponding to the production conditions (molding size shape, molding conditions, etc.).
Further, among molding conditions, particularly influence given to a surface quality by temperature and resin injection pressure is great. If a temperature of injected resin itself or a resin temperature heated by a mold is high, the resin viscosity reduces and the flowability of the resin increases, and although the impregnation property of the resin into the substrate is good, the flowability rapidly deteriorates by a high elevation rate of the viscosity, and when the molded product is big, there is a case where the flow of the resin reduces in speed on the way of the molding and it causes a non-impregnated portion. Even if the resin can flow over the entire area, in an area in which the viscosity has become high, there is a case where many voids and pinholes are generated even though non-impregnated portions are not generated. On the other hand, if there is an unevenness of the temperature of a mold or there is a change in the temperature during molding, there is a case where very fine gas bubbles remaining in the mold come into contact with each other and they grow a big bubble developing to a void or a pinhole.
Further, it is important that the pressure is also adequate. Namely, if the pressure is too high, the resin flow rate becomes high, and there is a case where it causes a disturbance of the weave structure of the substrate or it causes an expansion in volume in a cavity to generate bubbles, and if the pressure is too low, there is a case where residual bubbles cannot be compressed to be small.
Further, since a reactive gas may be generated from a reactive resin in its curing process, or fine gas (bubbles) having been contained in a resin may grow to voids or pinholes as the molding time passes, it is better to cure the resin as quickly as possible after the resin is impregnated into the substrate.
The influence given to the yield of the molding by the characteristics of the material of the reactive resin is very high, and for example, depending upon the kind of the curing agent, the reaction speed becomes maximum at an initial period of the reaction of the resin, and thereafter, the time passes. Therefore, the reaction speed reduces, and there is a case where the time required for the curing becomes long. On the contrary, if the curing time is to be shortened by elevating the temperature of the mold, there is a case where the initial viscosity increases too much, the viscosity is elevated too much at the time of resin injection and flow, ultimately the resin is gelated, and the molding is stopped on the way and a non-impregnated portion is generated.
Thus, in FRP molding (particularly, RTM molding), there exist proper molding conditions and material characteristic in accordance with molding size (area), and if not molded at proper conditions, problems on quality, in particular, on surface quality, are liable to occur.
Further, to improve the surface quality of a molded product as one of the purposes, a method is proposed wherein a reinforcing fiber substrate is given with a certain shape before it is disposed in a mold, by nipping it with upper and lower dies for preforming prior to RTM molding, and only the reinforcing fiber substrate preformed is disposed directly on the molding surface (for example, the aforementioned JP-A-2003-305719).
In such a conventional molding method, however, if a resin to be injected and cured is not delivered enough and is not impregnated into the details of the reinforcing fiber substrate, voids and pinholes may occur, and the mechanical properties of the molded product may be decreased, or the surface quality may be reduced. Especially, if voids or pinholes appear on the surface, in particular, on the design surface side, although usually patching such as charge of resin is carried out, this patching requires work and time, and decreases the efficiency of the whole of the manufacturing process.
As the countermeasure for preventing occurrence of such voids and pinholes injuring the design quality of the design surface, there is a case where a random mat layer is provided on the upper surface of a surface-layer substrate. This random mat layer is called as “a surface mat” because the random mat layer becomes an outermost layer, and particularly in a prepreg/autoclave curing method, an RFI (Resin Film Infusion) method, a hand-lay-up method, etc., it is sometimes employed. However, the structure thereof is a substrate structure in which the surface substrate and the random mat layer are completely replaced with each other, as compared the embodiment described later.
In a case where such a substrate structure is employed in a molding method such as RTM molding and vacuum molding wherein a resin fluid is injected into a dry substrate and flown and impregnated into the substrate, it is necessary to discharge also bubbles by the flow of the resin, and at a portion with a low resin flowability, voids are liable to be generated or pinholes are liable to occur by the left bubbles.
In a case where an FRP is molded by an RTM molding method or a vacuum molding method by using the above-described random mat as a surface mat and disposing it as an outermost layer, the random mat in a state of a dry substrate is pressed to the mold surface, and a gap between the mold surface and the random mat is very small because the bulkiness of the random mat with a low weight is low. Therefore, the resin flowability into the gap is poor, and as a result, voids and pinholes are liable to occur at the position thereof. Thus, particularly in an RTM molding method and a vacuum molding method, even if a random mat layer is provided as an outermost layer (a surface layer at a design surface), occurrence of voids and pinholes cannot be prevented.
Accordingly, paying attention to the above-described situations, it could be helpful to provide an RTM molding method and device wherein, even as for a relatively large three-dimensional configuration, the molding process from resin injection to impregnation and curing can be carried out at a high speed as compared with conventional RTM molding method and device, without generating non-resin-flowing areas, thereby achieving shortening of the molding time, increase of production speed and production amount, in particular, increase of production amount per one mold, and reducing the production cost.
Further, it could be helpful to provide an RTM molding method and device wherein, in an RTM molding for molding a relatively large fiber reinforced plastic product with a projection area of substantially 1 m2 or more, a voidless high-quality product can be molded efficiently in a short period of time.
Furthermore, it could be helpful to provide an RTM molding method wherein injected resin can be surely and easily delivered over the entire range of a desirable area in the resin injection step, and a fiber reinforced plastic with an improved surface quality can be produced by preventing occurrence of voids and pinholes on a surface, in particular, on the design surface side.